If it vibrates that easily, it may also flutter. For flutter, the best weight location is ahead of the hinge line, so that ~ 80% of the weight of the aileron is balanced. You can do this by putting a small weight on something like a control horn, so that the weight is ahead of the hinge line. For plain old vibration, you may be better off stiffening the structure, rather than adding weight to damp it out. It sounds like the aileron is already a bit too flexible, or inadequately supported, & extra weight may strain the structure.